This invention relates to axle differential mechanisms employed in vehicular interaxle or final drives, for achieving desired distributions of driving torque to the axles and associated wheels of a vehicle. More particularly, the invention relates to selectively actuated modes of axle differentiation within a single axle system.
The need for differentiation or variable distribution of driving torque to the driving axles of a vehicle has, of course, long ago been recognized and achieved. Thus, the conventional or unbiased differential allows for differences in angular velocities between opposed driving wheels upon the turning of corners. However, limiting differential action has also been achieved more lately for the purpose of avoiding the wheel spins whenever one of the driving wheels associated with an otherwise conventional differential encounters poor traction.
The usual devices for limiting differential action have generally utilized friction discs for the elimination of differential action until some predetermined safe amount of friction torque is exceeded, at which time the device permits differentiation.
The advantages and functions of a differential employing the combination of conventional or unbiased differentiation and biased differentiation are also well known and understood. For example, U.S. Pat. No. 3,448,635, issued to Nelson, discloses a dual mode differential mechanism containing both unbiased and biased differentiation modes, each selectively actuated by operator. By avoiding an otherwise continuously biased operation, a dual mode differential and mechanism provides for improved life of the friction disc devices employed within the differential. As biasing is not required during the majority of driving time, an overall improved clutch is achieved in that the clutch is rendered more effective and suitable over a wider range of design parameters.
It is further recognized, however, that although selection between unbiased and biased differential modes is beneficial, there are instances when a non-differentied or "solid axle" operation is both more safe and more desirable. Thus, for example, there are conditions when, in the limited slip mode, the differential input torque will cycle between values above and below the predetermined safe torque value of the limiting device. This action results in erratic "on and off" differential behavior, during which time the applied torque is shifted from one opposed driving wheel to the other, and can occasionally result in premature failure of the differential limiting device or devices.
Under such circumstances, it would be desirable to, at least momentarily, effectively avoid differential action entirely, preferably via the same control employed to select between conventional and biased differential action.